Why Does a Transition Metal Dichalcogenide Nanoribbon Narrow into a Nanowire under Electron Irradiation?

Transition metal dichalcogenide (TMDC) nanowires have practical applications in 1D electron channels, spintronics, optoelectronics, and catalysis due to their authentic subnanometer width (<1 nm) and intrinsic metallicity. Although narrowing of a TMDC nanoribbon into a nanowire under electron irradiation has been frequently observed in the synthesis of TMDC nanowires, the mechanism underlying this unexpected structural transformation remains a mystery. Here, to reveal the underlying mechanism, we combine first-principles calculations with a global structure search of 1D nanowires and show that a nanoribbon of 1H-phase MoS₂ with a width narrower than 6 rings is energetically unfavorable compared with its nanowire counterpart due to the edge-edge interaction. The bending effect induced by S defects under electron irradiation is the major driving force for the transition of MoS₂ nanoribbon into a nanowire. We predict that the precursor of the Mo₆S₆ nanowire is a well-defined Mo₁₁S₁₁-i nanowire with an unexpected stoichiometry. The intrinsic local compressive strain triggers a phase transition from Mo₁₁S₁₁-i to its slightly modified sister nanowire, Mo₁₁S₁₁-ii, which is characterized by the configuration (Mo₁S₁)₅&Mo₆S₆. Triggered by electron irradiation, the nanoribbon undergoes a step-by-step narrowing process with sequential peeling of a Mo₁S₁ fragment in each step to form a robust Mo₆S₆ nanowire. This unique narrowing mechanism is universal for the nanoribbon-to-nanowire transformation of other TMDCs under electron irradiation. Our study highlights a hitherto unexplored mechanism for creating individual M₆X₆ nanowires and contributes to an in-depth understanding of the narrowing of TMDC nanoribbons under electron irradiation.